Sudden changes in the power demands or changes in the system conditions in the power system are often followed by prolonged electromechanical oscillations leading to power system instability. AC transmission lines are dominantly reactive networks characterized by their per mile series inductance and shunt capacitances. Suitably changing the line impedance and thus the real and reactive power flow through the transmission line is an effective measure for controlling the power system oscillations and thereby improving the system stability.

Advances in high power semiconductors and sophisticated electronic control technologies have led to the development of FACTS. Through FACTS the effective line impedance can be controlled within a few milliseconds time. Damping of the power system oscillation is possible through effective changes in the line impedance by employing FACTS members (SVC, STATCOM, UPFC etc).